This invention relates to an improvement in or relating to a process for the production of high-density carbon materials directly from a green coke without the aid of a binder and to high-density carbon materials produced by such an improved process.
By the term "carbon materials" is meant both sintered and graphitized materials and the term "high-density" used with respect to the carbon materials means to have an apparent specific gravity of at least 1.75 when measured in graphitized form, the graphitization being effected at approximately 3000.degree. C.
High-density carbon materials are of importance in industry mainly as materials of machinery, electrodes for electro-discharge machinings, nuclear reactors and some others. Recently, because of their high corrosion resistance and high strength, particularly at high temmperatures, high-density carbon materials are being directed to new applications as corrosion-resistant structural materials and high temperature-resistant materials, for example. Thus, marked increase in the output of such high-density carbon materials may be expected.
Hitherto, high-density carbon materials were produced by long-established processes typically comprising the steps of finely pulverizing a calcined coke, incorporating a binder pitch having a softening point of about 80.degree.-110.degree. C. in an amount of 25-33% by weight based on the weight of calcined coke by kneading under heating, molding the kneaded mass by pressing in a mold, rubber-pressing, extrusion, isostatic pressing and the like, baking the molded mass at a temperature near 1000.degree. C. to form a carbonized mass having an apparent specific gravity of about 1.60-1.65, dipping under pressure the carbonized mass in a pitch melted at about 250.degree. C. and diluted with coal tar to impregnate the mass with the pitch, rebaking the impregnated mass at a temperature near 1000.degree. C. and repeating the pitch-impregnating and rebaking steps one or two times to obtain a carbon material having an apparent specific gravity of about 1.70-1.75 at most and, if desired, graphitizing the carbon material thus obtained at a temperature above 2800.degree. C. to obtain a graphite material having an apparent specific gravity of 1.8 or higher.
The old processes as above-mentioned are disadvantageous in that they require many steps, thus being very time-consuming. For example, more than one month is required from the pulverizing of calcined coke to the finishing of the graphite material even for the production of a small-sized product and more than three months are necessary for a large-sized product. Inevitably, therefore, the high-density carbon materials hitherto produced are expensive and the lowering of the production cost thereof will be difficult and unexpectable.
Various attempts have been made to find an improved process for the production of high-density carbon materials. Thus, there has already been developed on an industrial scale a process wherein a molded mass comprising a finely pulverized calcined coke kneaded with a binder pitch as prepared according to the old processes is baked at 700.degree.-900.degree. C. under an elevated pressure of 50-100 atmospheres for carbonization with the intention of enhancing the carbonization yield of the binder pitch whereby to obtain a carbon material having an apparent specific gravity of 1.7 or higher and, if desired, the carbon material thus obtained is graphitized at a temperature above 2800.degree. C. to yield a graphite material having an apparent specific gravity of 1.8 or higher. This process requires no pitch-impregnating and rebaking steps as required in the old processes, so that the time required can be reduced significantly. However, since the cost of high-pressure baking furnace is expensive both for construction and operation, this process is still unsatisfactory particularly from the economical point of view.
Recently, we have proposed a new type of process for the production of high-density carbon materials starting directly from a green coke, and not a calcined one. The new process comprises finely pulverizing a green coke containing a certain amount of volatile matters, molding the pulverized green coke as such, namely without the addition of a binder pitch, by pressing it in a mold and demolding and sintering the molded green coke at a sintering temperature to form a sintered carbon material and, if desired, graphitizing the sintered carbon material (see Japanese Patent Prepublication No. 150505/76 and Japanese Patent Application No. 155113/76). The formation of sintered carbon materials having a high apparent specific gravity and high mechanical strength from a green coke as such without the aid of a binder pitch is believed to result from such a so-called "self-sintering" phenomenon that firm carbon-carbon chemical bondings between coke particles are formed during the sintering where volatile matters contained in the green coke are decomposed and vaporized. It is expected that this process is available, in itself, as one of effectual processes for large scale production of carbon materials. However, this process has a serious disadvantage in that cracks are caused in the periphery of the molded green coke at the demolding stage which is effected by extrusion from the mold in which finely pulverized green coke has been molded by pressing. The cracks are mainly formed near the periphery of the molded green coke in perpendicular direction to the extrusion and in parallel with one another and are therefore so-called "lamination" or "laminar cracks". Hereinafter, we refer to such cracks as laminar cracks. We have found from our experience that laminar cracks are much more liable to be formed in larger-sized molds, making the production of homogeneous, larger-sized products difficult and, in some extreme cases, impossible. This disadvantage will be avoidable by the application of a mold release agent to the mold or by the use of a split mold, but the adoption of these means is apparently inadvisable because of low productivity, costliness of split mold and increase in the total production cost.
Our investigation was started on the assumption that the laminar cracks are caused by large friction between the coke particles and the inner wall of mold and with the intention of finding an effective antifriction aid to be incorporated in the powdered green coke. The first possibility for this purpose was to incorporate a small amount of a powdered solid lubricant into the powdered green coke. We firstly tried to incorporate a powdered graphite having a particle size below 200 mesh into a powdered green coke having a particle size below 10.mu. and found that the incorporation of about 3-5% by weight of the graphite was effective to a certain extent, but incomplete, on the prevention of laminar cracks and that the incorporation of more than 5% of the same resulted in a considerable decrease in the mechanical strength of sintered carbon materials derived therefrom. The incorporation of calcium stearate in an amount varying from 0.01 to 2% by weight based on the green coke was found to have little or no effect on the prevention of laminar cracks. The use of some other solid lubricants such as molybdenum disulfide and polycarbon monofluoride was also expected for this purpose, but was in fact inadvisable because of contamination of the resulting carbon materials with molybdenum and sulfur in the former case and of the generation of carbon fluoride or hydrogen fluoride gas during the baking stage in the latter case.
We then tried to examine the usefulness of a variety of liquid lubricants which would have no adverse effect on the sintering of green coke. As such liquid lubricants, we expected those to be useful which vaporize at such a slow rate that no appreciable cracks are formed in the molded green coke during the elevation of temperature up to about 400.degree. C. above which the sintering of green coke will start. From this point of view, we selected xylene and kerosene as typical examples of aromatic and aliphatic hydrocarbon solvents, respectively, with which a powdered green coke is easily wettable. Contrary to our expectation, however, the incorporation of 1-10% by weight of xylene had only a little effect on the prevention of laminar cracks and that of the same percentages of kerosene had no effect thereon at all.
The next step of our investigation was directed to the use of alcohols as another class of liquid lubricants with which a powdered green coke is wettable. We found that of alcohols, some of monohydric alcohols and all of polyhydric alcohols have little or no appreciable effect on the prevention of laminar cracks, whereas some others of monohydric alcohols are effective as antifriction aid for the purpose in question. Concretely, the incorporation of 1-15% by weight of a monohydric alcohol containing at least 4 carbon atoms and being in liquid state at room temperatures, such as butyl, octyl and benzyl alcohols was found effective for substantially reducing the frictional resistance caused on demolding the molded green coke from the mold by extrusion, thus resulting in the production of the molded green coke free from laminar cracks.
Our investigation was further continued to try the incorporation of a small amount of water into a powdered green coke instead of the monohydric alcohols above-mentioned. In fact, we supposed that there is little or no possibility for successful incorporation of water in view of such negative factors of powdered green coke as non-swelling with water, hydrophobic property and insufficient sintering in the presence of water. Quite unexpectedly, we have now found that powdered green coke can be wetted with a small amount of water relatively easily and that the presence of a certain small amount of water in the form of an intimate homogeneous mixture with a finely pulverized green coke to be molded is markedly effective for preventing laminar cracks, thus making it possible to produce large-sized carbon materials free from laminar cracks and having a high specific gravity and high mechanical strength.